I have ambivalent feelings about the necessity of updating the wiring
in an 80-year-old system.  If it was my piano I would do it if the
wiring was in bad shape, and I would also place a GFCI ahead of the
plug.  Ten bucks well spent if it might alert you to a problem before
it could become a big problem.

Assuming any ancient deteriorating wiring has been replaced and/or the
wiring is "like new", then I would recommend two things that will
enhance protection of the user and the instrument without altering it.

1) The GFCI as mentioned above, ahead of the plug.

2) It's likely that an original 80-year-old plug will not have the
larger tine which automatically insures that the hot side of the line
goes directly to the piano's main power switch, assuming everything is
wired correctly including the wall plug receptacle!  Use a two dollar
neon tester to verify the plug receptacle is correctly wired.  It is if
you get a light when you stick the tester in the smaller slot.

If you cannot even bear to replace the original plug, then verify which
wire/tine goes to the switch and put a small bit of red nail polish on
the tine screw or screw cover.  Be sure the red marked tine is plugged
into the smaller of the two plug receptacle slots.

Regarding: Ground vs. Earth

Actually there is no versus.  It's the same thing exactly.  It's called
a ground because it refers to _the_ ground, as in the planet's surface
you walk on, hence the earth.  Whenever you properly "ground" something
the path takes you back to earth by way of a grounding rod buried in
the earth.  The earth is actually used as a conductor for ground faults
in all commercial generating systems; has been from the beginning.

An example using a simple two conductor generator, a small plant
feeding a residence 100 feet away.  No transformers or complicated
stuff.  At the generating station, one side of the line, one conductor,
is grounded, grounded in the earth using a grounding rod.  The grounded
conductor (commonly referred to as the white wire) and the ungrounded
conductor (commonly referred to as the  "hot" wire) travel to the
residence.  At the main panel the grounded conductor is physically and
electrically connected to the panel.  A grounding wire connects the
grounded conductor, the main panel, the incoming cold water feed, and
a grounding rod buried in the earth.  All electrical tubing is to be
properly physically and electrically connected throughout the system
from the main panel on, so as to create a path to the earth ground from
any metal fixture in the system.

In a correctly wired and performing two wire system, as in the small
generating plant example, the grounded conductor (white) and the
ungrounded conductor ("hot") make up a complete circuit and carry all
of the load.  The grounding system is carrying no part of the load;
there is no movement of electrical energy through the grounding system.

However, should there be a fault in the circuit, then the grounding
system can act as a conductor to safely direct the fault to earth
ground.

Back to the question about the motor and the 3-wire plug.  As you
can see, adding a 3-wire conductor and plug to the motor does not make
for a grounded system.  The original power cord for the piano would
have to be replaced with a 3-wire cord and plug and the piano wired
appropriately to get the ground line to the new motor plug.  Of course
you still need a "modern" three hole plug receptacle into which to plug
the piano's power cord.

Many older homes still have only receptacles for the old 2-slot plug.
It would be ironic if after you went to the trouble to update the
original electrical system of the piano, you find that the customer,
who only has an ancient 2-slot plug receptacle, has used a plug adapter
to make the new 3-prong plug into a 2-prong, thus defeating the whole
exercise.  Then you discover that the plug receptacle is _really_ old
and doesn't even have the large and small slots, just two small slots.
The customer has therefore trimmed the larger tine of the plug adapter,
so there's now only a 50-50 chance of getting the hot wire to the
switch!

The customer is in the background wondering why the tech is mumbling
in an agitated way to himself and fiddling with the plug behind the
sideboard.  :o)

More on earth ground.  If you want physical proof that the earth is
part of modern generating systems, drive a rod or pipe into the earth.
About 10 feet of solid copper or brass rod preferred, but you can use
galvanized water pipe for this experiment.  Better if the ground is
damp.  Use a porcelain lamp holder, with a small lamp.  Attach a wire
between the lamp shell and the rod.  That would be your "white" wire
and the silver colored screw on the lamp holder.  Connect a 110 volt,
switched off, "hot" wire to the colored screw on the lamp holder.

(Do not stand on the wet earth near the rod in your bare feet.)  Stand
clear, flip the switch on, the lamp will light as the circuit is
completed between the "hot" wire and the earth-ground.  Do this only
momentarily as it shocks the earthworms and other beasties in the
immediate vicinity.  (My dad had a variation on this he used to bring
up earthworms from under our lawn for fishing bait.)  This exercise is
recommended only for the reasonably electro-mechanically inclined.
Otherwise, please just take my word for it.  ;o)

How a Ground Fault Circuit Interrupter (GFCI) works --

Just to confuse matters, the name is somewhat misleading.  It would
seem to indicate that the device would interrupt the path to ground.
This is never done in an electrical system.

The device could be rightly named a "Load Carrying Conductor Fault
Circuit Interrupter" but GFCI is a lot more elegant than LCCFCI.

In a standard two wire circuit, the GFCI monitors the current carrying
conductors.  If it detects a difference in the flow of energy between
them, it shuts down the circuit by interrupting the "hot" side of the
line.

Why?  Because if there is a difference between the flow through the
current carrying conductors, this would indicate that there is a leak
to ground somewhere in the circuit, which may be a shock hazard.

For instance, you are under a house, lying on wet earth using an
older ungrounded defective metal cased drill motor where some of the
electrical energy is getting to the drill case.  This will create an
imbalance between the two conductors as some of the energy from the
"hot" side of the line is bled off through the case, through your body
and then to the wet ground.  If you have a GFCI in the system, then you
may live to tell the tale, otherwise, who knows?  All depends on how
faulty the circuit is.

If the discussion about grounded and grounding is confusing maybe the
following will help.  example: Properly grounded two wire circuit with
white and black wire current carrying conductors.

White wire = grounded conductor (always white colored), black wire =
ungrounded conductor = "hot wire".  Grounding wire = plain copper wire
or green insulated wire.  (Green is only used for grounding wires,
never for the normal current carrying conductors).

Tom Lear
San Francisco, Calif.